A cited a pH value below 3 or above 9 is in line with the usually citation for the stability of the Al2O3 protective layer. Per a source on the effect of pH, to quote:
It is well known that aluminum resistance is related to the thin and compact layer of naturally formed oxide on aluminium surface, but this oxide layer is stable only in pH range 4-8. Lower or higher pH values caused prominent destroying of protective layer and thus the significant metal dissolution [15].
So, outside the pH range, your nano SiO2 coating formulation may be subject to attack. The presence of active Aluminum may serve to introduce some new chemistry.
Most likely, in my opinion, the unprotective Al can react even with water to form hydrogen and aluminum hydroxide. The latter reacts with so-called reactive silica per Science Direct:
Silica forms complex precipitates with iron, aluminum, and magnesium hydroxides.
which disrupts the SiO2.
Another but unlikely path starts with some atomic hydrogen ingressed into the Aluminum metal (see, for example, this Science Direct discussion and also a 2008 thesis "Alkaline dissolution of aluminum: surface chemistry and subsurface interfacial phenomena", by Saikat Adhikari discussing surface atomic hydrogen formation on Aluminum). Suggested reaction mechanics could then be if such a reaction is favorable (unlikely):
$\ce{.H ⇌ H+ + e-}$
$\ce{4 .H + SiO2 (nano) -(?)-> Si + 2 H2O}$
potentially directly destroying the nano SiO2. Here relatedly to the reaction above is a hydrometallurgy source discussing a commercial leaching application with $\ce{.H}$ on Al metal acting on $\ce{PbS}$ at the bottom of Page 218. However, the heat of formation of PbS is 10 times less than SiO2, hence this reaction path is not likely.
Proposed Solution: Test Copper vessels for suitability (see this reference).
